The synthesis and cyclotetramerisation reactions of aryloxy-, arylalkyloxy-substituted pyrazine-2,3-dicarbonitriles and spectroelectrochemical properties of octakis(hexyloxy)-pyrazinoporphyrazine

Article abstract of DOI:10.1016/j.dyepig.2009.12.003

Novel, aryloxy- and arylalkyloxy-substituted pyrazine dicarbonitriles were synthesized from 5,6-dichloropyrazine-2,3-dicarbonitrile and the corresponding phenol/alcohol derivatives. Cyclotetramerisation of these pyrazine derivatives to form metal pyrazino

Full text of DOI:10.1016/j.dyepig.2009.12.003

Dyes and Pigments 86 (2010) 115e122
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and cyclotetramerisation reactions of aryloxy-,
arylalkyloxy-substituted pyrazine-2,3-dicarbonitriles and spectroelectrochemical
properties of octakis(hexyloxy)-pyrazinoporphyrazine
,
Rabia Zeynep Uslu Kobak a, Egemen Selçuk Öztürk a, Atıf Koca b, Ahmet Gül a
*
a Department of Chemistry, Technical University of Istanbul, TR34469 Maslak, Istanbul, Turkey
b Department of Chemical Engineering, Marmara University, TR34722 Goztepe, Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel, aryloxy- and arylalkyloxy-substituted pyrazine dicarbonitriles were synthesized from 5,6-
dichloropyrazine-2,3-dicarbonitrile and the corresponding phenol/alcohol derivatives. Cyclotetramerisation
of these pyrazine derivatives to form metal pyrazinoporphyrazines in the presence of appropriate metal
salts in different solvents such as DMF, quinoline, 2-dimethylaminoethanol and n-hexanol, resulted in
decomposition products with the exception of teh latter solvent which lead to mainly octakis(alkyloxy)
pyrazinoporphyrazines. Cyclic voltammetry and differential pulsed voltammetry of the complexes
indicated that cobalt pyrazinoporphyrazine displayed both ligand and metal-based redox processes
while zinc and copper derivatives exhibited only ligand-based redox processes. The redox processes of
the pyrazinoporphyrazines shifted significantly towards positive potentials compared to those of the
common phthalocyanines. The novel compounds were characterised using elemental analysis and
spectral techniques.
Received 26 August 2009
Received in revised form
26 November 2009
Accepted 2 December 2009
Available online 22 December 2009
Keywords:
Diaminomaleonitrile
Pyrazine dicarbonitrile
Cyclotetramerisation
Pyrazinoporphyrazine
Electrochemistry
Cobalt
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
and enamines to give various substituted pyrazines [8]. Reaction
conditions can be altered by using triethylamine with alcoholates,
Dicyanopyrazine derivatives, which can be considered as
condensation products of diaminomaleodinitrile with -diketones,
pyridine in acetone with thiolates and sodium hydride in dioxane
with amines [9]. Thus, dicyanopyrazine derivatives can be used in
the design of a variety of functional materials such as tetrapyr-
azinoporphyrazines [10].
a
have been extensively investigated and are important intermedi-
ates in fields such as agricultural, food and medicinal chemistry
owing to their unique characteristics [1]. Recently, several
substituted dicyanopyrazine derivatives have been reported as
electroluminescent materials, synthetic reagents for coloring
materials and nonlinear optical materials [2]. When compared with
phthalonitrile derivatives, the special features of dicyanopyrazine
derivatives arise from the presence of two aza functions in the 1,4-
positions in the case of the pyrazine ring. Such special features
include an intense chromophoric effect, strong fluorescence (even
in solid state), high melting point and high solubility in polar
solvents. Owing to the strong electron withdrawing ability of the
5,6-dicyanopyrazine moiety, nucleophilic substitution at the 2- and
3-positions takes place using chloro-leaving groups. The dinitrile
compound readily reacts with various nucleophiles such as alco-
holates [3], thiolates [4], primary [5,6] and secondary amines [7]
Tetrapyrazinoporphyrazines which are formed by cyclo-
tetramerisation of pyrazine-2,3-dicarbonitriles or their derivatives
currently receive considerable attention because of their specific
optical properties that result from the 18-pi electron rich aromatic
macrocyclic structure, which can host various metal ions in its
central cavity [11]. Tetrapyrazinoporphyrazines can be used in most
of the areas where phthalocyanines are applied such as traditional
dyes and pigments, for controlling growth of microorganisms, as
electrocatalysts for oxygen reduction, materials for electrochromic
displays, media for optical recording, as photosensitive materials in
photodynamic cancer therapy, as optical filters and anti-copy inks
[12e20].
This paper concersn the synthesis of novel aryloxy- and ary-
lalkyloxy-substituted pyrazine dicarbonitriles. Using general
methods, cyclotetramerisation reactions were performed on these
pyrazine derivatives to form metal pyrazinoporphyrazines in
the presence of appropriate metal salts in different solvents such
as DMF, quinoline, n-hexanol, 2-dimethylaminoethanol and
* Corresponding authors. Tel.: þ90 212 285 68 27; fax: þ90 212 285 63 86.
E-mail address: ahmetg@itu.edu.tr (A. Gül).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.12.003

116
R.Z. Uslu Kobak et al. / Dyes and Pigments 86 (2010) 115e122
n-pentanol. Furthermore, the voltammetric and spectroelec-
trochemical properties of the hexyloxy tetrapyrazinoporphyrazines
were demonstrated.
procedure of the pyrazine dicarbonitriles except that DMF was used
instead of ethanol in the precipitation process. The yield of pale
yellow solid was 0.18 g (52.5%). IR, ymax (cmꢁ1): 3036 (CH,
aromatic), 2923 (CH, aliphatic), 2238 (C^N), 1445, 1345, 1146,
1064e1030 (CeO), 748, 692. MS(m/z): 341,874 [Mþ]. 1H NMR
2. Experimental
(CDCl3, d), ppm: 7.45e7.36 (m, 10H, AreH), 5.56 (s, 4H, OeCH2).
2.1. Equipment and materials
Calculated for C20H14N4O2: C 70.17; H 4.12; N 16.37%. Found: C
69.97; H 3.94; N 16.08%.
Electronic spectra were recorded on a Scinco SD-1000 double-
beam UVeVis spectrophotometer and IR spectra on a Perkin Elmer
Spectrum One FT-IR (ATR sampling accessory) spectrophotometer.
Elemental analyses were performed in the Instrumental Analysis
Laboratory of the TUBITAK Marmara Research Centre using Carlo
Erba 1106. 1H NMR spectra were recorded in CDCl3 solution using
a Bruker 250 MHz NMR spectrometer. Mass spectra were measured
on a Bruker Daltonics MicroTOF mass spectrometer. All starting
materials were purchased from major suppliers and used without
any further purification. Dimethylformamide was dried over 4 Å
molecular sieves and 5,6-dichloropyrazine-2,3-dicarbonitrile (1)
was obtained in a two step synthesis from diaminomaleodinitrile
(DAMN) according to procedures described in the literature [4].
2.2.2. General procedure of octakis(hexyloxy)
pyrazinoporphyrazinato metal (II) (6e8)
In order to synthesize the metal pyrazinoporphyrazine,
a synthesized pyrazine dinitrile (0.3 mmol of 2, 3, 4 or 5) was reacted
with the convenient anhydrous metal salt (zinc (II) acetate, cobalt
(II) chloride or copper (II) chloride (0.15 mmol)) in the presence of
10 m
L DBU and 1.2 mL of n-hexanol at 135 ꢀC for 6 h under nitrogen
atmosphere. After cooling, crude product was precipitated by
addition of MeOH (3 mL). The precipitate was filtered off, washed
with MeOH and acetone. The product was purified by column
chromatography on silica gel (SiO2) using different solvent or
solvent mixture as eluent.
2.2. Synthesis
2.2.2.1. Octakis (hexyloxy)pyrazinoporphyrazinato zinc (II) (6). This
compound was synthesized according to the general procedure of
octakis- (hexyloxy)pyrazinoporphyrazinato metal (II). The amounts
of the reagents used were as follows: 0.1 g of a dinitrile derivative 2
(0.25 mmol), 3 (0.3 mmol), 4 (0.22 mmol) or 5 (0.3 mmol) and
anhydrous zinc (II) acetate [(0.023 g, 0.125 mmol), (0.027 g,
0.15 mmol), (0.02 g, 0.11 mmol) or (0.027 g, 0.15 mmol) for 2, 3, 4, 5;
respectively]. The crude title compound isolated from the mixture
was then purified on silica gel column using methanol/chloroform
1:100 mobile phase. When 2, 3, 4 or 5 was used as the precursor,
yield was 0.024 g (27.5%); 0.028 g (26.9%); 0.014 g (18.4%); 0.022 g
(21.2%), respectively. IR, ymax (cmꢁ1): 2956e2858 (CH, aliphatic),
2.2.1. General procedure of pyrazine dicarbonitriles (2e5)
5,6-dichloropyrazine-2,3-dicarbonitrile 1 (0.2 g, 1 mmol) and
the corresponding hydroxyl derivative that is p-nitrophenol for 2
(0.28 g, 2 mmol), p-cresol for 3 (0.2 mL, 2 mmol), 3-(diethylamino)
phenol for 4 (0.33 g, 2 mmol), benzyl alcohol for 5 (0.2 mL, 2 mmol)
was stirred in 35 mL THF at 65 ꢀC. Reaction in the presence of
triethylamine (0.3 mL, 2.2 mmol) as a base required approximately
18 h to achieve the desired pyrazine dicarbonitriles. The reaction
mixture was filtered and the filtrate evaporated to dryness. The
residue was dissolved in ethanol and then precipitated by addition
of water; the yellow precipitate was collected by filtration and
washed with cold hexane and diethyl ether. The residue was further
recrystallized from acetone/hexane.
1638,1441,1120,1062 (CeO), 983,739. 1H NMR (CDCl3)
(m, 16H, OeCH2), 2.33e1.23 (br, 64H, CH2), 1.01 (br, 24H, CH3).
UVeVis lmax(nm) (log ) in CHCl3: 361 (5.34), 624 (5.11). Calculated
d: 4.44e4.20
3
for C72H104N16O8Zn:C 62.34; H 7.56; N 16.16%. Found: C 62.04; H
7.78; N 15.96%.
2.2.1.1. 5,6-Bis(4-nitrophenoxy)pyrazine-2,3-dicarbonitrile (2). The
yield of yellow solid was 0.32 g (78.8%). Melting point (m.p):
250e255 ꢀC. IR, ymax (cmꢁ1): 3114e3074 (CH, aromatic), 2240
(C^N), 1519, 1346, 1231 (AreOeAr), 1154, 851, 748. 1H NMR (d6-
2.2.2.2. Octakis(hexyloxy)pyrazinoporphyrazinato cobalt (II) (7). This
compound was synthesized according to the general procedure
employed for octakis (hexyloxy)pyrazinoporphyrazinato metal (II).
The amounts of the reagents were employed: 0.1 g of a dinitrile
derivative 2 (0.25 mmol) or 3 (0.3 mmol), anhydrous cobalt (II)
chloride [(0.016 g, 0.125 mmol) or (0.019 g, 0.15 mmol) for 2 and 3;
respectively]. The crude title compound isolated from the mixture
was then purified on silica gel column using first THF/chloroform
1:5 as the mobile phase and then once more with 2/100 methanol/
chloroform. When 2 or 3 was used as the ligand, the yield was
0.033 g (38.4%) and 0.037 g (36%), respectively. IR, ymax (cmꢁ1):
2956e2858 (CH, aliphatic), 1640, 1450, 1137, 1073 (CeO), 981,750.
acetone,
d), ppm: 8.50e7.67 (m, 8H, AreH). Calculated for
C18H8N6O6: C 53.47; H 1.99; N 20.79%. Found: C 53.51; H 2.05; N
20.84%.
2.2.1.2. 5,6-Bis(4-methylphenoxy)pyrazine-2,3-dicarbonitrile (3). The
yield of pale yellow solid was 0.27 g (79%). M.p: 235e242 ꢀC. IR,
ymax (cmꢁ1): 3036 (CH, aromatic), 2960e2925 (CH, aliphatic) 2238
(C^N), 1443, 1262e1238 (AreOeAr), 1154, 1017, 808. 1H NMR (d6-
acetone, d), ppm : 7.36e7.21 (m, 8H, AreH), 2.38 (s, 6H, CH3). MS(m/
z): 341,038 [Mþ]. Calculated for C20H14N4O2: C 70.17; H 4.12; N
16.37%. Found: C 70.35; H 4.26; N 16.16%.
MS: (m/z): 1380.9 [Mþ]. UVeVis lmax(nm) (log
3) in CHCl3: 334
(5.26), 608 (5.09). Calculated for C72H104N16O8Co: C 62.64; H 7.59; N
16.23%. Found: C 62.98; H 7.87; N 16.55%.
2.2.1.3. 5,6-Bis[3-(diethylamino)phenoxy]pyrazine-2,3-dicarbonitrile
(4). The yield of pale yellow solid was 0.31 g (67%). M. p:
137e139 ꢀC. IR, ymax (cmꢁ1): 3078 (CH, aromatic), 2974e2934 (CH,
aliphatic) 2236 (C^N), 1613, 1446, 1270e1232 (AreOeAr), 1129,
2.2.2.3. Octakis(hexyloxy)pyrazinoporphyrazinato copper (II) (8). This
compound was synthesized according to the general procedure
used for octakis (hexyloxy)pyrazinoporphyrazinato metal (II). The
amounts of the reagents were used as follows: 0.1 g of a dinitrile
derivative 2 (0.25 mmol) or 3 (0.3 mmol), anhydrous copper (II)
chloride [(0.017 g, 0.125 mmol) or (0.02 g, 0.15 mmol) for 2 and 3;
respectively]. The title compound isolated from the mixture was
then purified on silica gel column using methanol/chloroform
2:100 as the mobile phase. When 2 or 3 was used as ligand, yield
1021, 760. 1H NMR (d6-acetone,
d), ppm: 7.30e6.48 (m, 8H, AreH),
3.45e3.36 (q, 8H, NeCH2), 1.18e1.08 (t, 12H, CH3). Calculated for
C26H28N6O2: C 68.40; H 6.18; N 18.41%. Found: C 68.12; H 6.46; N
18.16%.
2.2.1.4. 5,6-Bis(benzyloxy)pyrazine-2,3-dicarbonitrile (5). Synthesis
and purification were carried out as outlined for the general

R.Z. Uslu Kobak et al. / Dyes and Pigments 86 (2010) 115e122
117
was 0.04 g (46.51%) and 0.054 g (52.43%), respectively. IR, ymax
(cmꢁ1): 2955e2858 (CH, aliphatic), 1638, 1446, 1133, 1067(CeO),
phenoxy substituents in quinoline at 160 ꢀC, similar conversion of
aryl- or arylalkyl-oxy-substituted pyrazine-dicarbonitriles 2e5
prepared during this work to Azapcs could not be realized either in
the presence of metal ions or without any templating metal ion
[22,23].
982, 745. UVeVis lmax(nm) (log 3) in CHCl3: 345 (5.38), 621 (5.48).
MS: (m/z): 1384.83 [Mþ]. Calculated for C72H104N16O8Cu: C 62.43; H
7.57; N 16.18%. Found: C 62.17; H 7.23; N 15.93%.
Changing the reaction conditions such as reaction temperature
or solvents (e.g. DMF, 2-dimethylaminoethanol, etc.) did not lead to
formation of any porphyrazines. A cyclotetramerisation reaction
occurred when n-hexanol and a hindered base 1,8-diazabicycl-
oundec-7-ene (DBU) were used. However, this case trans-
etherification took place in parallel in the basic medium and the
products no longer carried the phenoxy-substituents but the hex-
yloxy-groups from the solvents instead. It should be considered as
a reaction in line with the transesterification observed in the case of
phthalocyanines with ester substituents [24]. Consequently, with
the presence of DBU in n-hexanol 2, 3, 4 or 5 were reacted with zinc
(II) acetate, cobalt and copper (II) chloride to form octakis(hex-
2.3. Electrochemical analysis
Cyclic voltammetry (CV), differential pulse voltammetry (DPV),
and double potential step coulometry (DPSC) measurements were
carried out with Gamry Reference 600 potentiostat/galvanostat
controlled by an external PC and utilizing a three-electrode
configuration at 25 ꢀC. The working electrode was a Pt disc with
a surface area of 0.071 cm2. The surface of the working electrode
was polished with a diamond suspension before each run. A Pt wire
served as the counter electrode. Saturated calomel electrode (SCE)
was employed as the reference electrode and separated from the
bulk of the solution by a double bridge. Ferrocene was used as an
internal reference.
yloxy)pyrazinoporphyrazinato metal complexes 6,
7 and 8,
respectively (Fig. 1).
Electrochemical experiments were performed in dry dichloro-
methane (DCM) containing 0.1 moldmꢁ3 electrochemical grade
tetrabutylammoniumperchlorate (TBAP) as a supporting electro-
lyte. The solutions were bubbled with high purity N2 to remove
dissolved O2 at least 15 min prior to each run and an N2 atmosphere
was maintained above the solution during the measurements. For
each measurement the reference electrode tip was moved as close
as possible to the working electrode so that uncompensated
resistance of the solution was a smaller fraction of the total resis-
tance, and therefore the potential control error was low. Moreover,
IR compensation was applied to the CV scans to further minimise
the potential control error.
The spectroelectrochemical measurements were carried out
with an Ocean-optics QE65000 diode array spectrophotometer
equipped with the potentiostat/galvanostat utilizing a three-elec-
trode configuration of thin-layer quartz spectroelectrochemical cell
at 25 ꢀC. The working electrode was light penetrating Pt tulle. Pt
wire counter electrode separated by a glass bridge and an SCE
reference electrode separated from the bulk of the solution by
a double bridge were used.
Except for 6 (ca. 22% yield), compounds 7 and 8 were synthe-
sized in relatively moderate yields in 36e52% range. These new
octakis(hexyloxy)pyrazinoporphyrazinato metal complexes, 6, 7
and 8 were purified by column chromatography on silica gel using
methanol and chloform as the eluent. They show high solubility in
most organic solvents such as chloroform, dichloromethane,
tetrahydrofuran, acetone and pyridine.
All new compounds were identified through several spectro-
scopic techniques including FT-IR, FAB-Ms, 1H NMR, UVeVis and
elemental analysis. The spectroscopic data of the ligands were in
accord with their assigned structures. Pyrazinoporphyrazines
exhibited different vibrations from the substituent groups. Despite
the pyrazine 2, which did not have any aliphatic CH group in its
structure, the stretching vibrations of aliphatic CH group for all the
products 6, 7 and 8 were observed as sharp and intense absorption
bands appearing in 2956e2858 cmꢁ1. The absorption band at
around 3070 cmꢁ1 was characteristic for the aromatic CH group.
This band disappeared upon conversion to 6, 7 and 8 although all of
these pyrazines contain an aromatic CH. In addition, the sharp peak
at 2238 cmꢁ1 disappeared after conversion into the pyr-
azinoporphyrazines in support of the cyclotetramerisation reaction
of pyrazines. According to these results, IR spectral data of 6, 7 and 8
indicate the formation of alkyloxy-pyrazinoporphyrazines.
3. Result and discussion
3.1. Synthesis and characterization
In the case of zinc pyrazinoporhyrazine 6, 1H NMR spectra
indicated aliphatic CH2, CH3 and OeCH2 protons which are incon-
gruous with the expected results. These data prove the presence of
the unexpected hexyloxypyrazinoporphyrazine. Furthermore, in
the mass spectrum molecular ion peaks were noted at m/z 1380.9
for 7 which confirmed incorporation of the hexyloxy unit.
Electronic absorption spectra of all pyrazinoporphyrazines 6, 7
and 8 were analyzed in chloroform and all of them showed Q bands
around 620 nm and B bands around 345 nm as expected for alky-
loxy-substituted pyrazinoporphyrazines (Fig. 2) [25].
5,6-dichloropyrazine-2,3-dicarbonitrile 1, which is also avail-
able commercially [21], was derived from diaminomaleodinitrile
DAMN and oxalyl chloride in a two step synthesis according to
procedures described in the literature [4]. Compound 1 was treated
with p-nitrophenol, p-cresol and 3-(diethylamino)phenol as
phenol derivatives and benzyl alcohol as an arylalcohol in THF at
65 ꢀC. Reaction in the presence of triethylamine as a base required
approximately 18 h to afford 2, 3, 4 and 5 in moderate yields; the
only difference being in the latter where the product was precipi-
tated by addition of water to its DMF solution. All of these pyrazine
products were yellow or yellowish solids.
3.2. Voltammetric and spectroelectrochemical measurements
Using typical methods, cyclotetramerisation reactions were
carried out on these pyrazine derivatives to obtain the desired
metal pyrazinoporphyrazines with the appropriate metal salt in
different solvents such as DMF, quinoline, n-hexanol, 2-dimethy-
laminoethanol, n-pentanol as a high boiling solvent, in the pres-
ence of DBU as a catalyst. However from the range of solvents used
only hexanol afforded products which were subsequently charac-
terised as the hexyloxypyrazinoporphyrazines. Although in some
recent papers, it was reported that metal-free azapcs were
prepared from pyrazine-dicarbonitrile derivatives with bulky
The solution redox properties of the complexes (6e8) were
studied by using CV, DPV and DPSC techniques in DCM on a plat-
inum electrode. Table 1 lists the assignments of the couples
recorded and the estimated electrochemical parameters, which
included the half-wave peak potentials (E1/2), and DE1/2. Table 1 also
represents E1/2 values of related MPc and pyrazinoporphyrazine
(PyPz) complexes in the literature for comparasion [26e33]. The
main difference of redox processes between the complexes 6e8
and the complexes reported in the literature are the peak potentials
of the redox couples. All couples generally shifted approximately

118
R.Z. Uslu Kobak et al. / Dyes and Pigments 86 (2010) 115e122
Fig. 1. Synthesis of Pyrazinoporphyrazines (i) p-nitrophenol, triethylamine, THF; (ii) p-cresol, triethylamine, THF; (iii) 3-(diethylamino)phenol, triethylamine, THF; (iv) benzyl
alcohol, triethylamine, THF; (v) metal salts and/or DBU, Hexanol, 6 h, 135 ꢀC.
0.20e0.40 mV towards positive potentials with respect to the MPcs
having similar substituents [26e36]. These potential shifts of PyPz
are due to the electron donating ability of the nitrogen atoms
placed instead of carbons in the macrocyclic core of phthalocyanine
analogues. Moreover, while the first oxidation process of CoPc
complexes in a non-polar solvent is a ligand-based process, it is
metal-based process for CoPyPz (7). As shown on the Table 1, DE1/2
values of MPyPz complexes studied here are slightly larger than
that of the complexes reported in the literature. All complexes are
aggregated in solution at high dilution. It is well documented that
the introduction of nitrogen atoms to macrocyclic core can increase
aggregation properties of the Pc complexes [37]. CV and DPV
studies reveal that while CoPyPz (7) gives five well defined metal
and/or ring-based redox couples, ZnPyPz (6) and CuPyPz (8) give
only ring-based redox couples within the potential windows of the
DCM/TBAP electrolyte and Pt electrode system.
spectroelectrochemistry measurements. As shown in Fig. 3, the first
reduction and oxidation couples are split into two peaks. This
behavior is due to the aggregation of the complex in solution.
Dilution of the solution changes the aggregationedisaggregation
equilibria of the complex (inset in Fig. 3A). When the solution is
diluted the peak current of the wave III’ assigned to the aggregated
species became smaller than that of III, however it is present even
in very diluted solution. The wave II’ gives a small reverse couple
during the reverse scan supporting the aggregation tendency of the
complex.
Spectroelectrochemical measurements (SE) were used to
confirm some of the assignments in the CVs. The experiments were
performed using optically transparent thin-layer quartz electro-
chemical cell. Fig. 4A shows the in situ UVevis spectral changes
during the controlled potential reduction of 7 at ꢁ0.55 V vs. SCE.
There are two distinct changes during this potential application.
First of all, while the intensity of the absorption of the Q band
corresponding to the neutral [CoIIPyPzꢁ2] species decreases with
a red shift from 606 to 612 nm, a new band corresponding to the
Fig. 3 shows the CV and DPV of 7 in DCM containing TBAP.
Complex 7 gave three one-electron reduction processes labelled as
III at ꢁ0.05 V (III’ at ꢁ0.28 V), IV at ꢁ1.13 V, and V at ꢁ1.89 V and
two one-electron oxidation processes labelled as II at 1.25 V (II’ at
0.85 V) and I at 1.65 V versus SCE at 0.100 Vsꢁ1 scan rate. The
assignments of the redox couples are confirmed below using
electro-generated reduced [CoIPyPzꢁ2 ꢁ1
species at 495 nm
]
appears. Then while the intensity of the absorption of the Q band
increases with red shift from 612 to 618 nm and the band at 495 nm
increases in intensity, two new bands at 370 and 655 nm appear. At
the same time the broadness of the Q band, due to aggregation,
decreases during this potential application. The spectral changes in
Fig. 4A are typical of metal-based reduction in porphyrin
complexes. The shift of the Q band from 608 to 618 nm and the
observation of new bands at 495 and 655 nm (MLCT) are charac-
teristic of metal-based processes and assigned to [CoIIPyPzꢁ2]/
[CoIPyPzꢁ2 ꢁ1
]
[36,38e43]. The process at ꢁ0.55 V potential appli-
cation occurred with clear isosbestic points at 305, 366, 545, 611
and 681 nm in the spectra. The color of the solution changed from
blue to red during the process. The reversion to the original spec-
trum after the potential application at 0.00 V indicates the
reversibility of the process. During the controlled potential reduc-
tion of 7 at ꢁ1.30 V vs. SCE corresponding to the redox process
labelled IV (Fig. 4B), the absorptions for Q bands at 618 nm
decreases in intensity without shift and the bands at 495 and
655 nm increase in intensity, while the band at 370 nm decreases in
Fig. 2. UVeVis spectra of compounds 6e8 (CHCl3, 10ꢁ6 M).

R.Z. Uslu Kobak et al. / Dyes and Pigments 86 (2010) 115e122
119
Table 1
Voltammetric data of the complexes with the related complexes from the literature
for comparison.
Complex
Redox processes
I
IIb
IIIc
IV
V
fDE1/2 Ref
ZnPyPz
(6)
CoPyPz
(7)
E1/2
e
1.36e
ꢁ0.53
ꢁ1.05 ꢁ1.73e 1.89 tw
ꢁ1.13 ꢁ1.88e 0.90 tw
ꢁ1.10 ꢁ1.75e 1.63 tw
ꢁ1.03 ꢁ1.25 0.53 26
ꢁ1.06 ꢁ1.90 1.10 27
ꢁ0.88 ꢁ1.38 0.90 28
(V vs. SCE)
aE1/2
1.65e 0.85
ꢁ0.05
(V vs. SCE)
E1/2
(V vs. SCE)
E1/2 (SCE)
in THF
(1.25)d (ꢁ0.28)d
CuPyPz
(8)
1.50e 0.97
ꢁ0.56
ꢁ0.21
ꢁ0.95
ꢁ0.29
ꢁ0.91
ꢁ0.75
ꢁ0.40
ꢁ0.99
ꢁ0.73
ꢁ0.24
ꢁ0.27
gCoPc
0.76 0.32
0.15
0.92 0.61
hCuPc
iCoPc
iCuPc
jZnPc
kCoPc
lZnPc
E1/2 (SCE)
in DMSO
E1/2 (Fcþ/Fc)
in DCM
e
E1/2 (Fcþ/Fc)
in DCM
e
e
0.56
0.85
ꢁ1.27
ꢁ1.16
ꢁ0.84
e
e
e
e
e
1.47 28
1.60 29
0.82 30
1.47 31
E1/2 (SCE)
in DCM
E1/2 (Ag/AgCl) 0.89 0.42
in DCM
E1/2 (SCE) in
1.16 0.48
1.03 0.65
1.11 0.66
DMF
mZnPyPz E1/2 (SCE)
in DCM
ꢁ0.94 ꢁ1.23 1.38 32
ꢁ0.58 ꢁ0.97 0.89 33
mCoPyPz E1/2 (SCE)
in DCM
nCuPyPz E1/2 (SCE)
in DCM
e
e
ꢁ0.66 ꢁ1.34
e
33
tw: this work.
a
E1/2 ¼ (Epa þ Epc)/2 at 100 mVsꢁ1 (Epc for reduction, Epa for oxidation for irre-
versible processes).
b
This redox couple is a metal-based oxidation process for the complex 7.
This redox couple is a metal-based reduction process for the complex 7.
dEp of the wave of aggregated species.
Recorded by differential pulse voltammetry.
c
d
e
f
D
E1/2
¼
D
E1/2 (first oxidation) ꢁ E1/2 (first reduction) ¼ HOMOeLUMO gap for
D
metal-free and metallophthalocyanines having electro-inactive metal center (metal
to ligand (MLCT) or ligand to metal (LMCT) charge transfer transition gap for MPc
having redox active metal center.
Fig. 3. A) CVs of CoPyPz (7) (5.0 10e4 moldmꢁ3) at various scan rates on Pt in DCM/
TBAP Inset: CVs CoPyPz (7) at various concentration at 0.100 Vs-1 scan rate on Pt in
DCM/TBAP). B) DPVs of CoPyPz (7) (5.0 10e4 moldmꢁ3) (pulse time: 50 ms; pulse size:
100 mV, step size: 5 mV, sample period: 100 ms).
g
Substituted with tetrakisdiethoxymalonyl groups.
Substituted with propane 1,2-diolsulfanyl groups.
Substituted with tetra(dihexoxymalonyl) groups.
Substituted with tetrapentafluorobenzyloxy groups.
Substituted tetrakis (benzylmercapto) groups.
Substituted tetra-{2-(2-thienyl)ethoxy groups.
Substituted with tetrakis-2,3-(5,7-diphenyl-1,4-diazepino) groups.
Substituted with tetrakis-2,3-[5,6-di(2-pyridyl) pyrazino groups.
h
i
j
k
l
m
documented in literature that the first oxidation process of CoPc
complexes is a ring-based processes in noncoordinating electrolyte
systems such as DCM/TBAP, however this process is metal centered
in the case of azaphthalocyanine, also called pyrazinoporphyrazine.
Based on the CV, DPV, SE and CPC (discussed below) data, the
following mechanism eq. (1) may be proposed for the reduction
and oxidation processes of the complexes:
n
intensity. The spectral changes in Fig. 4B are typical of ligand-based
reduction assigned to [CoIPyPzꢁ2 ꢁ1/[CoIPyPzꢁ3 ꢁ2
] ] redox process
[36,38e43]. The process at ꢁ1.30 V vs. SCE potential application has
isosbestic points at 330, 390, 549, 637 and 697 nm in the spectra.
The color of the solution is changed from red to purple during the
process.
-e
[CoIIIPyPz-1]+2
[CoIIIPyPz-2]+
I
On potential application at 1.50 V, the intensities of the Q bands
increases with a red shift from 608 to 617 nm, while two new bands
appear at 360 and 442 nm (Fig. 4C). At the same time while the B
band shifts from 335 to 325 with a decrease in intensity, the broad
Q band becomes sharp due to the disaggregation of the aggregated
species during the process. The spectral changes in Fig. 4 C, espe-
cially shifting of the Q band with increasing intensity is typical of
a metal-based oxidation in phthalocyanine and porphyrazine
complexes and thus the process is easily assigned to [CoIIPyPzꢁ2]/
II
-e
II'
-e
[CoIIPyPz-2]
monomeric species
[CoIIPyPz-2]
aggregated species
(1)
III
+e
III'
+e
+e
[CoPyIPz-4]-3
+ e
IV
[CoIPyPz-2]-1
[CoIPyPz-3]-2
[CoIIIPyPzꢁ2 þ1
redox process [38e43]. The process has isobestic
]
V
points at 313, 350, 376, 406, 510, 596, and 637 nm in the spectra and
blue to green color changes upon the potential application at 1.50 V.
Return to the original color and spectrum after the potential
application at 0.0 V shows the reversibility of the process. It is well
.
Complexes 6 (Fig. 5) and 8 give very similar voltammetric
behavior accompanied by the slightly potential shifts corresponding

120
R.Z. Uslu Kobak et al. / Dyes and Pigments 86 (2010) 115e122
Fig. 5. A) CVs of ZnPyPz (6) (5.0 10e4 moldmꢁ3) at various scan rates on Pt in DCM/
TBAP (Inset: CVs ZnPyPz (6) at various concentrations at 0.100 Vs-1 scan rate on Pt in
DCM/TBAP). B) DPVs of ZnPyPz (6) (5.0 10e4 moldm-3) (pulse time: 50 ms; pulse size:
100 mV, step size: 5 mV, sample period: 100 ms).
the splitting of the peaks due to the existence of aggrega-
tionedisaggregation equilibrium in solution.
The complexes 6 and 8 both have redox inactive metal centers.
Therefore, in situ UVevis spectral changes of 6 are given as
a representative of the spectral chances of MPyPz having redox
inactive metal center. Fig. 6 shows the in-situ UVeVis spectral
changes of 6 during the potential applications at the redox poten-
tials. As shown in Fig. 6A, change of the intensity of the Q band at
618 nm without the wavelength shifting during the first reduction
process of the complex and observation of new bands in the MLCT
regions at 526, 706, 786 and 869 are characteristic of ring-based
processes. During the process the Q band becomes sharper through
decrease of the intensity of the shoulders of the Q band at 587 nm.
At the same time, the B band at 330 nm decreases in intensity. The
process has isobestic points at 294, 366, 400, 480, 540, and 654 nm
in the spectra. Fig. 6B shows the spectral chances during the second
ligand-based reduction of the complex under applied potential at
ꢁ1.30 V. Decrease in the intensity of Q band at 618 nm and
observation of the new bands at 524 and 865 nm in the MLCT
regions are characteristics of ring-based processes. Fig. 6C shows
the ligand-based oxidation processes of the complex under the
potential applications at 1.50 V. Decreasing of the intensity of Q
band at 618 nm, and observation of the new bands at 390 and
720 nm in the MLCT regions are characteristics of ring-based
processes [36e41].
Fig. 4. In-situ UVevis spectral changes of CoPyPz (7). A) Eapp
Eapp ¼ ꢁ1.30 V. C) Eapp ¼ 1.50 V.
¼ ꢁ0.55 V. B)
to the different metal center of the complexes (Table 1). Both
complexes give three reduction redox couples and one oxidation
process (two for 8). Both complexes have a tendency to aggregate in
solution even at very low concentration, a feature similar to
complex 7. The values of Ipa/Ipc, D
Ep, and Ip/y1/2 change with the
concentration of the complex and the scan rate due to the aggre-
gationedisaggregation equilibria present in solution [17,21]. As
shown in Fig. 5, the redox couples of 6 are split into two peaks. This
behavior is due to the aggregation of the complex in solution.
Dilution of the solution changes the aggregationedisaggregation
equilibria of the complex (inset in Fig. 5A). While the solution is
diluted, peak current of the waves assigned to the aggregated
species (II’ and III’) became much smaller than those of the waves
corresponding to the monomeric species (II and III). Aggregation is
also effective during the second reduction process (IV). At higher
concentration, a new wave starts to appear just negative of the
potential of the wave IV. Moreover, the process IV shifts to poten-
tials that are more negative. DPV measurements (Fig. 5B) represent

R.Z. Uslu Kobak et al. / Dyes and Pigments 86 (2010) 115e122
121
potentials due to the nitrogen atoms of the pyrazine parts of the
complexes, which can be favorable for catalytic applications.
Acknowledgements
This work was supported by the Research Fund of the Technical
University of Istanbul.
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